Loudspeakers or speakers transform electrical signals into acoustical energy. Many loudspeakers have a transducer, sometimes referred to as an active driver, a driver, or a passive radiator, mounted in a speaker enclosure. The speaker enclosure may have a box-like configuration with sides and a back enclosing the transducer. The speaker enclosure may have other shapes and configurations including those that conform to environmental conditions of the loudspeaker location, such as in a vehicle. The transducer may provide a full range of acoustical frequencies from low to high. The transducer may provide a particular range of acoustical frequencies, such as low frequencies and/or midrange frequencies. Many loudspeakers have multiple transducers and/or a combination of transducers in the speaker enclosure. When multiple transducers are utilized in the speaker enclosure, it is common for individual transducers to operate in different frequency bands.
A transducer generally may have a cone connected along its outer perimeter to a frame by a surround. The cone may be made of paper, polymer, metal, ceramic, composites, and the like. The frame may be made of metal or other rigid material. The surround may be made of an elastomer like foam rubber, a doped cloth, or other pliable material and can contribute to isolating the cone from the frame. The cone is connected along its inner perimeter to a former, which is wrapped with insulated wire to form a voice coil. The voice coil generally is positioned within the magnetic gap of a magnetic field generated by one or more permanent magnets and may move in a linear fashion inside the magnetic field. The motor structure, generally including one or more permanent magnets, may be attached to the frame. When an electric potential or voltage is passed through the voice coil, the wire windings of the voice coil generate an electromagnetic field that interacts with the magnetic field of the one or more permanent magnets. This magnetic interaction results in a force being applied to the voice coil. This force moves the former, causing the cone to vibrate or oscillate. This vibration or oscillation of the cone can produce acoustical energy, such as a sound wave.
Low frequency transducers (“woofer”), midrange frequency transducers (“midrange”), and high frequency transducers (“tweeter”) generally produce less acoustical energy as the frequency decreases. Woofers and midranges generally may have a cutoff frequency where the acoustical energy drops about 3 dB below the average energy produced by a given transducer. At frequencies below the cutoff frequency, the acoustical energy produced by the transducer generally decreases rapidly.
A speaker system that includes a transducer, such as a woofer and/or a midrange, may be equipped with a bass-reflex device, such as a vent, port, or passive radiator, to extend the low frequency (bass) response of the system. A bass-reflex device can be tuned or configured to operate at or below the cutoff frequency for the transducer. This resonance of the bass-reflex device may contribute to the total acoustical output of the loudspeaker by extending the low frequency output below that of a sealed system. A loudspeaker with a bass-reflex device may have an extended bass response, thus allowing it to produce lower frequencies than a sealed system with a similar transducer arrangement. A bass-reflex device often is located on the same side of the loudspeaker enclosure as the transducer. The bass-reflex device also may be located on other sides of the loudspeaker enclosure.
A bass-reflex device generally uses the acoustical energy or air pressure generated within an enclosure to extend the low frequency response of the system. When the voice coil of a transducer moves in the magnetic gap, the former may move the cone toward the interior of the enclosure. This movement generates an acoustical wave in the interior of the enclosure. This acoustical wave cannot emanate from the loudspeaker if the enclosure is sealed. The acoustical energy associated with this acoustical wave generated within the enclosure generally is “lost” in the loudspeaker enclosure. A bass-reflex device can use this acoustical energy to resonate below and/or at the cutoff frequency of the transducer.
Some loudspeakers use a port as the bass-reflex device. A port may be a tube-like opening in the speaker enclosure. The port generally is “tuned”—sized and configured—to resonate the air column within the port at a frequency at and/or below the cutoff frequency of the transducer. The air column within the port resonates in response to acoustical energy generated within the enclosure. The resonance frequency of the air in the port may be limited by the available air volume in the speaker enclosure and is often difficult to control. Other loudspeakers may use a passive radiator as the bass-reflex device. A passive radiator generally is a rigid body mounted within an opening in the speaker enclosure. The rigid body is connected to the speaker enclosure by a surround. The rigid body may be made of paper, polymer, metal, composites, or other noncompliant materials. The surround generally is made of foam rubber, doped cloth, an elastomer, or other pliable material.
A passive radiator translates the air pressure created by the transducer into movement or resonance of the rigid body within the surround. The resonance of the rigid body can generate acoustical energy at a frequency below the cutoff frequency of the transducer. The mass and compliance of the rigid body can control the resonance frequency of the passive radiator. The rigid body may have a conic, flat, or other shape. A passive radiator may look like another “transducer” except without the voice coil, magnet, and related components. In some applications, such as vehicle and in-wall mounting, there may be little or no available space for a transducer and a passive radiator in the speaker enclosure to perform at the highest efficiency, and having a cost-effective design.